This invention relates to the field of devices for non-destructively testing electrically conductive materials and, in particular, to an eddy current sensor for non-destructive testing of the quality of electrically conductive through-hole plating in printed circuit boards.
The invention can be employed in machine building industry, instrument making, computer technology and electronics to test the quality of electrically conductive hole wall platings, inner tube surfaces, electrically conductive through-hole plating in printed circuit boards and other hollow objects with electrically conductive inner surface.
There is known a mutual-inductance coupling probe device for testing the integrity of through-hole plating in printed circuit boards (cf. U.S. Pat. No. 3,840,802, Cl.224-37, filed 1974). This sensor comprises two magnetic structures which are separated by electrically conductive shields. The magnetic circuits are provided with windings and are made so that their pole faces are turned towards the wall of the tested hole and located at an angle of 90.degree. apart. The winding of one of the magnetic circuits serves to excite an alternating magnetic flux in the wall of the tested hole, whereas EMF is induced in the winding of the second magnetic circuit which depends on eddy currents in the hole wall plating and, consequently, on the quality of this wall plating (thickness of the electrically conductive plating, its specific conductivity, various defects).
The sensitivity of the known sensor in measuring the thickness of an electrically conductive plating is low, as well as its testing output capacity, and the manufacturing process is rather complicated. Low accuracy of measurements derives from the fact that the EMF of the measuring winding is to a substantial degree a function of the distance between the butt ends of the magnetic circuits and the walls of a tested hole. That is why considerable errors are introduced by radial motions of the sensor within the hole.
Poor output capacity of testing can be accounted for by the fact that the sensor probe is to be very accurately located within the hole with the electrically conductive plating-to-be-tested and then moved along the plated wall.
Since the holes in printed circuit boards have diameters of the order of 0.5-2 mm, manufacturing of a probe device containing electrically conductive shields and intricately shaped magnetic circuits of strict sizes becomes a hard task to cope with.
There is also known an eddy current sensor for non-destructive testing of the quality of electrically conductive through-hole plating in hollow objects.
This eddy current sensor comprises an exciting and two opposing measuring windings which can slide along a magnetic circuit rod. This permits increase of the voltage balance level of the measuring windings before measurements by changing the distance between the exciting and measuring windings. When the maximum possible level of voltage balance of the measuring windings is attained by shifting the windings, they should be fixed on the magnetic circuit rod.
When alternating current is run through the exciting winding, an alternating magnetic field is produced. Its lines of force propagate along the magnetic circuit axis and close in the air. Then the mutual coupling between corresponding measuring and exciting windings depends upon the distance therebetween. By moving windings in relation to each other, a high degree of voltage balance can be attained in measuring windings. In case the magnetic circuit is placed in the hole space or at its entrance, eddy currents are induced in the electrically conductive plating which redistribute the flux between the main magnetic flux in the rod and the leakage flux.
The known sensor accuracy is low, as well as its output capacity for testing electrically conductive plating in holes of printed circuit boards.
The above described sensor may be used for testing the quality of electrically conductive through-hole plating in printed circuit boards as follows. If a tested printed circuit board is placed between the exciting and measuring windings, miniature measuring windings should be manufactured with a diameter less than the diameter of the tested hole. This makes the process of manufacturing an eddy current sensor complicated, and reduces its reliablility and testing output capacity. It is also possible that the tested printed board is placed at the tip of the magnetic circuit or that the magnetic circuit with windings secured thereon is introduced into the tested hole so that a corresponding measuring winding is at the moment of measurement between the exciting winding and the tested printed circuit board. When the measuring windings and the magnetic circuit are positioned in relation to the hole in this way, the signal of the eddy current sensor is highly dependent on the distance between the corresponding winding and the test object. Besides, axial movement of the measuring sensor with windings in relation to the test object is required before measurements followed by its fixation, which also reduces the testing output capacity.
The object of the present invention is to increase the accuracy of testing the quality of electrically conductive through-hole plating in printed circuit boards.